Cardiac tissue elasticity sensing

ABSTRACT

A system and method are provided for assessing the compliance of internal patient tissue for purposes of catheter guidance and/or ablation procedures. Specifically, the system/method provides for probing internal patient tissue in order to obtain force and/or tissue displacement measurements. These measurements are utilized to generate an indication of tissue elasticity. In one exemplary embodiment, the indication of elasticity is correlated with an image of the internal tissue area and an output of this image including elasticity indications is displayed for a user.

CROSS REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/146,881 having a filing date of Jan. 3, 2014, which is a continuationof U.S. patent application Ser. No. 13/413,235 having a filing date ofMar. 6, 2012 and which is issued as U.S. Pat. No. 8,644,950, which is adivisional of U.S. patent application Ser. No. 11/845,250 having afiling date of Aug. 27, 2007 and which issued as U.S. Pat. No. 8,131,379on Mar. 6, 2012, the entire contents of each of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The instant invention is directed toward an electrode catheter and amethod for using the electrode catheter for tissue mapping, guidanceand/or tissue ablation. In particular, the electrode catheter of thepresent invention may assess tissue elasticity in-vivo to facilitatecatheter guidance and/or ablation.

b. Background Art

Catheters have been in use for medical procedures for many years.Catheters can be used for medical procedures to examine, diagnose, andtreat while positioned at a specific location within the body that isotherwise inaccessible without more invasive procedures. During theseprocedures a catheter is typically inserted into a vessel near thesurface of the body and is guided to a specific location within the bodyfor examination, diagnosis, and treatment. For example, catheters can beused to convey an electrical stimulus to a selected location within thehuman body, e.g., for tissue ablation. Catheters with sensing electrodescan be used to monitor various forms of electrical activity in the humanbody, e.g., for electrical mapping.

Catheters are used increasingly for medical procedures involving thehuman heart. Typically, the catheter is inserted in an artery or vein inthe leg, neck, or arm of the patient and threaded, sometimes with theaid of a guide wire or introducer, through the vessels until a distaltip of the catheter reaches the desired location for the medicalprocedure in the heart. In the normal heart, contraction and relaxationof the heart muscle (myocardium) takes place in an organized fashion aselectro-chemical signals pass sequentially through the myocardium.

Sometimes abnormal rhythms occur in the heart, which are referred togenerally as arrhythmia. The cause of such arrhythmia is generallybelieved to be the existence of an anomalous conduction pathway orpathways that bypass the normal conduction system. These pathways can belocated in the fibrous tissue that connects the atrium and theventricle.

An increasingly common medical procedure for the treatment of certaintypes of cardiac arrhythmia is catheter ablation. During conventionalcatheter ablation procedures, an energy source is placed in contact withcardiac tissue (e.g., associated with a anomalous conduction pathway) tocreate a permanent scar or lesion that is electrically inactive ornoncontractile. The lesion partially or completely blocks the strayelectrical signals to lessen or eliminate arrhythmia.

Ablation of a specific location within the heart requires the preciseplacement of the ablation catheter within the heart. Precise positioningof the ablation catheter is especially difficult because of thephysiology of the heart, particularly because the heart continues tobeat throughout the ablation procedures. Commonly, the choice ofplacement of the catheter is determined by a combination ofelectrophysiological guidance and computer generated maps/models thatmay be generated during a mapping procedure. Accordingly, it isdesirable that any map or model of the heart be as accurate aspracticable.

Several difficulties may be encountered, however, when attempting toform lesions at specific locations. One such difficulty is obtainingaccess to specific areas of the heart. For instance, access to the leftatrium and pulmonary veins often requires performing a transseptalprocedure where a catheter or other instrument is pushed through theinteratrial septum between the left and right atriums. Such aninstrument preferably punctures the septum at its thinnest location, forexample the fossa ovalis. This location is not readily determined usingconventional imaging techniques such as fluoroscopy or intracardialmapping. Instead, the physician determines the puncture location basedon his/her experience using the electrode catheter to probe theinteratrial septum to identify the most compliant location, typicallythe fossa ovalis. Such experience only comes with time, and may bequickly lost if the physician does not perform the procedure on aregular basis.

Another difficulty is selecting the proper amount of ablation energy toform a lesion. In this regard, the energy required to form a lesion of adesired dimension relates to several factors, including the forceapplied by the electrode to the tissue. Such force is dependent upon thecompliance of the tissue, which may be a function of the thickness ofthe tissue. However, tissue compliance is not readily determined usingconventional imaging techniques.

BRIEF SUMMARY OF THE INVENTION

It is often desirable to be able to assess the compliance of internalpatient tissue for purposes of catheter guidance and/or ablationprocedures. Accordingly, it has been determined that the compliance or‘elasticity’ may be determined by probing internal tissue and measuringthe resistive force of the tissue and/or the displacement of the tissue.

According to one aspect, a system and method for performing a medicalprocedure is provided. The system/method includes positioning a catheterat a first position relative to a first tissue location of an internaltissue area. A first force measurement is then obtained as indicative ofa first force between the catheter and the first tissue location. Thisfirst force measurement may then be utilized to generate a tissueelasticity value that is associated with the first tissue location. Oncesuch an elasticity value is determined, a medical procedure may beperformed based at least in part on this information.

Such medical procedures include, without limitation, utilizing one ormore tissue elasticity values to determine a location to puncture aninternal tissue area. For instance, a region of the tissue area havingthe lowest elasticity value may correspond with the thinnest section ofthe tissue area. For instance, this area may correspond to the fossaovalis. Accordingly, a catheter or another device may puncture thislocation in a transseptal procedure. In another arrangement, theelasticity values for the tissue area may be utilized to select ablationpowers and/or contact forces for use during ablation procedures.

Positioning the catheter relative to the internal tissue area mayinclude identifying an initial contact position where the catheterinitially contacts the tissue at the first location. Positioning mayfurther entail displacing the electrode catheter a first distancerelative to the initial contact location, for example, to the firstposition. The electrode catheter may be displaced a second distancerelative to the initial contact location to a second position to obtaina second force measurement. In one arrangement, the first and secondforce measurements, as well as the first and second displacement values,may be utilized to generate the elasticity value. In furtherarrangements, additional displacements and force measurements may beobtained.

In one arrangement, a robotic actuator may be utilized to displace thecatheter relative to the initial contact location. In such anarrangement, the robotic actuator may provide controlled displacementand/or output displacement values for use in determining elasticity. Inanother arrangement, a catheter displacement sensor may be utilized tomeasure catheter displacement.

The system/method may further entail positioning the catheter at aplurality of tissue locations and generating a tissue elasticity valuefor each location. In this regard, the elasticity of the internal tissuearea may be mapped. Such mapping may allow for the identification ofstructures of interest such as the fossa ovalis and/or previous ablationareas within the internal tissue area. That is, differences inelasticity may be used for the identification of structures of interest.

In one arrangement, stored data indicative of predetermined elasticityvalues may be utilized to generate an elasticity value for the force.That is, the force measurement may be utilized with, for example,predetermined look-up tables, equations, or other data (e.g.,collectively catalog data) to determine an elasticity value for a tissuelocation. In another arrangement, the force measurement may be compareddata values in put by an operator. That is, the measured force value maybe compared to values set by an operator based on the experience of theoperator.

The system/method may further entail generating a display output of theinternal tissue area. Such a display output may provide a visualindication of the differing elasticity values for different locationregions of the internal tissue area. For instance, modeling of thedifferent elasticity values may be performed in order to generate asurface model based on the elasticity of the tissue area. Further, areasof different elasticity may have different visual indications (e.g.,colors) to allow a user to visually identify areas of interest.Accordingly, such an output may be utilized for catheter guidance,ablation procedures and/or other procedures.

Determining force may include measuring an output of a transducer thatis connected to a contact surface of the catheter. Determining force mayfurther entail obtaining force and direction information. That is, thedirectional information may be indicative of a contact angle between thecontact surface of the catheter and the tissue. Such force and directionmeasurements may be obtained utilizing a three-axis transducer. However,it will be appreciated that single-axis transducers may be utilized aswell. In further arrangements, force measurements may be equated toimpedance measurements. In such a system, the catheter may include oneor more electrodes that contacts the tissue. Accordingly, by measuringthe impedance an indication of the force of the contact the electrodeand tissue may be generated.

According to another aspect, a system/method for performing a medicalprocedure is provided. The system/method includes providing an imagingsystem for generating an image of an internal tissue area of interest.The system/method further entails probing the internal tissue area toobtain an indication of tissue elasticity for at least a first locationwithin the tissue area. Based on the indication of tissue elasticity,the image of the internal tissue area may be altered to provide a visualindication of the tissue elasticity at the first location. That is,tissue elasticity values may be correlated with an image of an internaltissue area.

Probing may entail contacting the internal tissue area utilizing acatheter. Accordingly, the catheter may be displaced against the tissueat the first location to obtain a first force measurement and may beadvanced relative to the first position to obtain a second forcemeasurement. Accordingly, these first and second force measurements anddisplacements associated with the first and second tissue displacementsmay be utilized to generate the indication of tissue elasticity.

The foregoing and other aspects, features, details, utilities, andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary catheter system,which may be implemented to access internal patient tissue for mappingand/or tissue ablation procedures.

FIG. 1 a is a detailed illustration of the patient's heart in FIG. 1,showing the electrode catheter after it has been moved into thepatient's heart.

FIG. 2 is an exemplary introducer and catheter that may be utilized withthe system of FIG. 1

FIGS. 3A and 3B illustrate catheter placement within a patient's heart.

FIGS. 4A, 4B and 4C illustrate advancement of a catheter relative tointernal patient tissue.

FIG. 5A illustrates a plurality of tissue contact locations andelasticity values as disposed on an image of an internal tissue area.

FIG. 5B illustrate a graphical representation of elasticity values ascorrelated with an image of an internal tissue area.

FIG. 6 illustrates a functional diagram of a system for determiningtissue elasticity.

FIG. 7 illustrates a protocol for use in determining internal tissueelasticity.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of a catheter system and methods of using thesystem to measure tissue elasticity are depicted in the figures. Asdescribed further below, measuring tissue elasticity, as provided by thepresent invention, provides a number of advantages including, forexample, the ability to locate a structure(s) of interest based on theelasticity of the structure and/or the elasticity of surroundingstructure. Measuring tissue elasticity also allows for determining forceapplied to tissue, which may allow enhanced control of lesiondimensions.

FIG. 1 is a diagrammatic illustration of an exemplary electrode cathetersystem 10 which may be implemented to access and map internal patienttissue and/or perform medical procedures on such tissue. Further, thesystem 10 is operative to assess tissue elasticity to assist in tissuemapping, catheter guidance, ablation procedures and/or other procedures.The catheter system 10 may include a guiding introducer having a sheath8, which may be inserted into the patient 12. The sheath 8 may provide alumen for the introduction of a catheter 14 which may be disposed beyondthe distal insertion end of the sheath 8. During an exemplary procedure,a user (e.g., the patient's physician or a technician) inserts thesheath of a guiding introducer into one of the patient's blood vessels18, e.g., through the leg (as shown in FIG. 1) or the patient's neck.The user, guided by an imaging device (e.g., fluoroscopy, ICE,electro-anatomical mapping, etc.) moves the sheath 8 into the patient'sheart 16 (as shown in more detail in FIG. 1 a).

When the sheath 8 of the guiding introducer is positioned in a desiredlocation within the heart 16 of the patient, the electrode catheter 14may be extended through a lumen of the sheath 8 such that the electrodecatheter 14 may be guided to a desired location within the heart toperform, for example tissue mapping and/or tissue ablation. In tissuemapping procedures, a model of the heart may be generated on an outputdisplay 11, which may be utilized for subsequent catheter guidance. Oneor more additional catheters 14 a may also be utilized during mappingand/or subsequent procedures. In addition, a robotic actuator 60 may beutilized to partially or completely control catheter movement.

FIG. 2 illustrates one embodiment of a guiding introducer system 36 withan electrode catheter 14 that may be selectively extended from thedistal end portion of the sheath 8. As used herein and commonly used inthe art, the term “distal” is used generally to refer to components ofthe catheter system, such as the tip electrode 20, located toward theinsertion end of the of the ablation catheter 14 (i.e., toward the heartor other target tissue when the catheter is in use). In contrast, theterm “proximal” is used generally to refer to components or portions ofthe catheter that are located or generally orientated toward thenon-insertion end of the catheter (i.e., away from or opposite the heartor other target tissue when the catheter is in use).

The sheath 8 is a tubular structure defining at least one lumen orlongitudinal channel. The sheath 8 is used to introduce and guide thecatheter 14 to a targeted internal tissue area. The catheter 14,however, may be used alone or with other guiding and introducing typedevices depending on the particular procedure being performed. As shownin FIG. 2, the catheter includes a tubular body or shaft 6 extendingfrom a proximal handle 4, through the sheath 8 and extending out of thedistal end of the sheath 8. The proximal handle 4 or a proximal portionof the shaft 6 may be connected to a robotic actuator. In one exemplaryembodiment, the sheath 8 and shaft 6 are fabricated with a flexibleresilient material. The sheath and the components of the catheter arepreferably fabricated of materials suitable for use in humans, such aspolymers. Suitable polymers include those well known in the art, such aspolyurethanes, polyether-block amides, polyolefins, nylons,polytetrafluoroethylene, polyvinylidene fluoride, and fluorinatedethylene propylene polymers, and other materials. In the particularablation system configuration of FIG. 2, the sheath 8 is configured toreceive and guide the catheter 14 within an internal lumen to a locationin the heart once the sheath is pre-positioned in an appropriatelocation.

The electrode catheter 14 of the exemplary embodiment includes a tipelectrode 20 and a force transducer 22. The force transducer 22 isoperative generate an output that is indicative of a force applied tothe tip electrode 20 or to other elements attached to the end of theelectrode catheter (e.g., needles, transducers, etc). Non-limitingexamples of force transducers that may be utilized include pressuretransducers and strain gages. Exemplary transducers that may be utilizedare variously disclosed in U.S. patent application Ser. No. 11/647,314,filed 29 Dec. 2006 entitled “Pressure-sensitive conductive compositecontact sensor and method for contact sensing”; U.S. patent applicationSer. No. 11/647,316, filed 29 Dec. 2006 entitled “Pressure-sensitiveconductive composite electrode and method for ablation”; U.S. patentapplication Ser. No. 11/647,279, filed 29 Dec. 2006 entitled “Design ofablation electrode with tactile sensor”; U.S. patent application Ser.No. 11/647,294, filed 29 Dec. 2006 entitled “Contact-sensitivepressure-sensitive conductive composite electrode and method forablation”; U.S. patent application Ser. No. 11/549,100, filed 12 Oct.2006 entitled “Dynamic contact assessment for electrode catheters”; andU.S. patent application Ser. No. 11/553,965, filed 27 Oct. 2006 entitled“Systems and methods for electrode contact assessment”, the entirecontents of which are incorporated by reference herein.

In one exemplary embodiment, the force transducer is a three-axistransducer that is operative to generate vector information. In thisregard, if the tip electrode 20 is contacted with a tissue wall, theforce applied to the tip electrode 20 as well as the angle the tipelectrode 20 contacts the wall may be determined. In another embodiment,the force transducer may be incorporated at the proximal end of thecatheter. Further, in robotic systems, motor current representingapplied motor torque may be calibrated to yield an applied force at thecatheter tip. It will be appreciated other electrical parameters of themotor/robotic system may also be utilized to generate an indication ofapplied force.

The tip electrode 20 and/or electrodes of another catheter 14 a may beimplemented to electrically map the myocardium (i.e., muscular tissue inthe heart wall). In this regard, information from the electrode(s) maybe utilized to create realistic model of cardiac chamber geometries ormodels of other internal tissue depending on the particular procedurebeing performed. Such a model may be displayed on a user output 11 (SeeFIG. 1) for use in catheter navigation, for example, during an ablationprocedure performed after mapping.

To create the model, any appropriate positioning/location system thatallows for internal mapping and modeling can be used. One such exemplarypositioning/location system is the NavX endocardial mapping systemproduced by St. Jude Medical Inc. In one embodiment, two or moreexternal patient electrode patches 46 (only one shown) are applied ontwo or more locations on the body. An electrical signal is transmittedbetween the patches 46, and one or more electrodes of one or morecatheters 14 within the heart sense the signal. The system 10 collectselectrical data from the catheter(s) 14 and uses this information totrack catheter movement and construct three-dimensional (3-D) models ofthe heart chamber in which the catheter is positioned. Additionally aphysician may sweep the catheter(s) 14 across the heart chamber duringdata collection to outline the structures and relay the signals to thecomputer system, which generates the 3-D model. The resulting model maythen be utilized to, for example, guide the catheter 14 to one or morelocations in the heart where treatment is needed. During mapping, it maybe desirable that one of the electrode catheters 14 and/or 14A include aplurality of electrodes for receiving electrical signals. Further, suchelectrodes may be disposed, for example, in a spiral pattern such thatthere is a three dimensional displacement of these receiving electrodes.In this arrangement, data gathered by the dispersed three-dimensionalarray of electrodes may allow for improved mapping of the interior of,for example, a cardiac chamber. Such a system allows for the creation ofdetailed internal models at the time of study and/or performance of aninternal procedure. This is, the system may be operative to generatesubstantially real-time models.

During catheter ablation, a physician advances the catheter 14 to targettissue utilizing the model. An ablation electrode, for example, tipelectrode 20, is then maneuvered to contact targeted tissue. Current isthen applied to the catheter electrode 20. This current passes throughthe catheter electrode 20, through patient tissue and back to theexternal electrode patch 46 on the body surface of the patient. Themyocardium around the catheter electrode 20 is heated by Joule effect.When the myocardial temperature exceeds a predetermined threshold (e.g.,50° Celsius) the tissue loses electrical excitability. Stated otherwise,application of electrical energy creates a lesion within the targetedtissue. Accordingly, accurate mapping of the tissue is necessary toproperly locate target tissue and create lesions to prevent undesiredelectrical activity.

In addition to accuracy of mapping, success of the ablation procedure isdependent upon effective lesion formation. Generally, the size of alesion is affected by a number of physiological and catheter dependentvariables. For instance, catheter tissue contact, catheter geometry,blood flow around ablation site, applied power, duration, andtemperature of the ablation electrode all affect lesion creation.Another variable that affects the creation of a lesion is the forceapplied by the electrode to the tissue. Applied force is related to themechanical compliance or ‘elasticity’ of the tissue. Accordingly,knowledge of the elasticity of the tissue prior to lesion formation maypermit correct application of force to the tissue during an ablationprocedure. Further, the elasticity of the tissue may change over atissue area of interest. Knowledge of changes in tissue elasticity mayalso allow for variable force application during lesion formation.

The knowledge of tissue elasticity may facilitate additional proceduresas well. For instance, identification of the elasticity of the differentareas within a heart chamber may allow for guiding a catheter to adesired location. In one exemplary arrangement, tissue elasticity mayfacilitate transseptal catheter guidance. As may be appreciated, cardiacaccess is often provided by inserting the sheath 8 in the femoral veinin the right leg. The sheath is then maneuvered up to the inferior venacava 52 and into the right atrium 54. See FIG. 3A. In what is typicallyreferred to as a transseptal approach, the sheath and/or a catheterextending through the sheath is passed through the interatrial septum 24between the right atrium 54 and left atrium 56. This provides access tothe left atrium 56 as well as the pulmonary veins.

Such transseptal access may be achieved by positioning the sheath 8 atthe appropriate location in the right atrium 54 and extending a dilatorand a needle (not shown) through a lumen of the sheath 8. When thedilator and needle are within the sheath 8, the ablation catheter 14 maybe removed from the sheath 8. In an exemplary transseptal procedure, theneedle is pressed through the interatrial septum 24 between the rightand left atriums. Following the needle, the dilator is pressed throughthe small opening made by the needle. The dilator expands the openingsufficiently so that the sheath 8 may then be pressed through theopening to gain access to the left atrium 56 and the pulmonary veins.With the sheath in position, the dilator is removed and the catheter 14is extended into the lumen of the sheath 8 and pushed along the sheathinto the left atrium 56. See FIG. 3B. When the catheter 14 is positionedin the left atrium 56, various procedures, such as mapping and ablationmay be performed therein.

It will be noted that the location where the needle or other puncturingdevice passes through the interatrial septum 24 is of importance.Specifically, it is desirable that the device pass through the fossaovalis. Found in the right atrium of the heart, the fossa ovalis is anembryonic remnant of the foramen ovale. In the fetal heart, the foramenovale allows blood to enter the left atrium from the right atrium. Itallows blood entering the right atrium to bypass the pulmonarycirculation. In most individuals, the foramen ovale closes within thefirst year after birth to form the fossa ovalis. However, while beingclosed in most patients, the fossa ovalis remains considerably thinnerthan surrounding structure of the septum. Further, a puncture in thefossa ovalis will readily close in most patients.

Some experienced physicians are able to determine the location of thefossa ovalis in the interatrial septum by feel. That is, the physiciandetermines location of the fossa ovalis based on his/her experienceusing a catheter 14 to probe the interatrial septum 24 and identify thelocation having the greatest compliance (e.g., the thinnest location).Such experience only comes with practice, and may be quickly lost if thephysician does not perform the procedure on a regular basis. In anycase, the number of doctors that can perform such a procedure islimited. Accordingly, the ability to measure the mechanical complianceof internal patient tissue such as myocardium may allow for improvedcatheter guidance as well as improved lesion formation.

The mechanical compliance or elasticity of the myocardium or otherinternal tissue may be determined utilizing the catheter 14incorporating the force transducer 22 as discussed above. Suchelasticity may be determined by probing the patient tissue and measuringthe force between a contact surface of the catheter (e.g., the tipelectrode 20) and patient tissue and the displacement of the patienttissue. Such force and displacement measurements may be utilized togenerate an elasticity value for a tissue location. As illustrated inFIG. 4A, the procedure begins by contacting the tip electrode 20 withthe patient tissue. In this exemplary embodiment, the tissue ismyocardial tissue of the interatrial septum 24. As will be appreciated,initial contact may be identified utilizing, for instance, impedancemeasurements. In this regard, the difference of resistivity betweenblood and myocardium tissue 24 causes an impedance change as the tipelectrode 20 contacts the myocardium 24. This impedance change may beutilized to identify a position of initial contact, which may beutilized as a reference position.

Once initial contact is made, the electrode 20 may be displaced a firstdistance x₁ (e.g., relative to the reference position) to a firstposition. See FIG. 4B. At this time, the tissue 24 is compressed by thetip electrode 20. Generally, cardiac/myocardial tissue exhibitsviscoelastic characteristics. In this regard, it shows a partialrelaxation of force after a constant depth insertion is applied.However, the tissue also exhibits an elastic reaction force thatprovides a constant resistive force against the electrode 20 thatremains after the tissue partially relaxes. Accordingly, the forcetransducer 22 may be utilized to measure this constant resistive force,for example, after the partial relaxation of the tissue. The resistiveforce f₁ and tissue displacement x₁ may be recorded. After this firstmeasurement is made, the tip electrode 20 may be advanced to a seconddistance x₂ to a second position. See FIG. 4C. Again, the forcetransducer 22 may be utilized to measure the resistive force applied tothe electrode tip 20 by the tissue. This second force f₁ and secondtissue displacement x₂ may also be recorded. In this regard, two forcemeasurements and two tissue displacement values are recorded for asingle tissue location. It will be appreciated that additionalincremental advancements of the tip electrode 20 and correspondingmeasurement by the force transducer 22 may be made. Of note, it may bedesirable to obtain the different force measurements at a common pointin the cardiac cycle. For instance, QRS or P wave gating may beimplemented to assure that measurements are obtained at common cardiaccycle times.

The process of obtaining force and displacement measurements may berepeated at a plurality of different locations for a tissue area ofinterest. For instance, a plurality of probing measurements may be madeon the interatrial septum. As will be discussed herein, such pluralityof measurements may be utilized by a modeling system to identifyelasticity of different areas of the septum.

Of note, it may be desirable to utilize the robotic actuator 60 toadvance the catheter 14 and, hence, tip electrode 20 against the tissue24. In this regard, it will be appreciated that such robotic actuationmay provide for better movement control of the catheter as well asautomated recording of tissue displacement. For instance, such roboticactuation may allow for sub-millimeter advancement. However, it will beappreciated that hand advancement may be utilized as well. For instance,markings may be provided on proximal portions of the sheath 8 and/or theshaft 6 of the catheter 14 that may provide an indication ofdisplacement of the catheter to the sheath. Alternatively, a sensor maybe utilized to monitor the position of the catheter relative to thedistal end of the sheath. What is important is that a tissuedisplacement measurement is provided for each force measurement at eachtissue location that is probed. Additional force and displacementmeasurements for each tissue location may increase the accuracy of aresulting elasticity determination.

As noted, cardiac/myocardial tissue exhibits at least partially elasticcharacteristics. Generally, the elastic characteristic of the tissue inresponse to a constant depth insertion follows a second order polynomialthat approximates the relationship between the force and the penetrationdepth. Specifically:

F _(elastic) =K ₁ x+K ₂ x   Eq. (1)

where F is the force as measured by the force transducer 22 and x is thepenetration depth (i.e., tissue displacement). In this regard, the K₁and K₂ values are representative of the elasticity of a probed tissuelocation. Such coefficients may be derived from two or more force anddisplacement measurements for a common tissue location. Accordingly, foreach probe location, where at least two force and displacementmeasurements have been obtained, K₁ and K₂ may be obtained. In any case,upon determining K₁ and/or K₂, the probe location may be assigned anelasticity value. Such a value(s) may be applied to an output image/mapgenerated by the system 10.

Further, stored data indicative of predetermined elasticity values maybe utilized to generate an elasticity value for a force and/ordisplacement measurement. That is, a force measurement, a displacementmeasurement, or both may be utilized with, for example, predeterminedlook-up tables, equations, or other data (e.g., collectively catalogdata) to determine an elasticity value for a tissue location. As will beappreciated, such stored data or catalog data may be determinedclinically in animal and/or human trials/testing. In anotherarrangement, an operator (e.g., physician) may input data values intothe system 10 (see FIG. 1). Force and/or displacement values may then becompared to the input data values. In this regard, the operator mayinput expected values associated with the elasticity, force and/ordisplacement for one or more internal regions (e.g., the fossa ovalis).That is, the measured values (force and/or displacement) may be comparedto values set by an operator based on the experience of the operator.

FIG. 5A illustrates an internal tissue area of interest as may bedisplayed on an output 11 of the system 10. In this exemplaryembodiment, the area of interest is the interatrial septum 24. As a userprobes the septum, an indication may be generated on the imageindicating a tissue location that has been probed. As shown in FIG. 5A aplurality of tissue locations 70 a-n within the tissue area may beprobed an indication of each probed location may be correlated with theimage. Further, a tissue elasticity value may be illustrated with eachprobed location. Upon generating an elasticity value for each probedtissue location 70, a graphical representation of the elasticity for thetissue area 24 may be generated. See FIG. 5B.

FIG. 5B illustrates modeling the elasticity values (e.g., K₁ and/or K₂)for different tissue locations 70 graphically. That is, the elasticityvalues may be surface modeled to provide, for example, a contour maprepresentation of the elasticity for the probed tissue area 24. Asshown, the region of lowest elasticity may correspond with the fossaovalis 72, which is the thinnest structure within the interatrial septum2 and has the lowest elasticity value K₁. That is, due to the thinnessof this tissue, it provides little elastic resistance to displacement.In any case, tissue eleasticities may be correlated with an image inorder to provide a visual indication of different elasticity values fordifferent tissue locations. Accordingly, a user may utilize this imageto identify, for example, the fossa ovalis for performing a transseptalprocedure. As will be appreciated, by providing such a graphicalrepresentation, a user (e.g., physician) may not rely entirely on theirsense of feel to identify a transseptal access location in theinteratrial septum.

Additionally, such information may be utilized for ablation purposes. Asnoted above, lesion size is determined in part by the force applied tothe tissue. As will be appreciated, thinner areas of tissue may deflectmore for a given displacement and therefore there may exist less forcebetween an electrode and the tissue. Accordingly, the force between theelectrode and the tissue may be adjusted in order to create a lesionwith desired characteristics. In this regard, an elasticity map, such asshown in FIG. 5B, may also allow for adjustment of catheter-tissuecontact force during an ablation procedure to provide desired lesioncharacteristics.

Furthermore, the elasticity values may be utilized to provide a warningto an operator. For instance, a warning (e.g., auditory and/or visual)may be issued to an operator based on the determination of theapplication of an excessive force in relation to a tissue elasticityvalue for a tissue location. Such an excessive force determination maybe made, in one embodiment, by monitoring a ratio or a change in a ratioof elasticity (K) and force. Such warnings may be utilized to reduce thelikelihood of unintended tissue puncturing.

FIG. 6 is a functional block diagram showing the system 10 in moredetail as it may be implemented to determine the elasticity of tissuecontacted by the catheter 14. It is noted that some of the componentstypical of conventional tissue ablation and/or mapping systems are shownin simplified form and/or not shown at all in FIGS. 1 and 6 for purposesof brevity. Such components may nevertheless also be provided as partof, or for use with the catheter system 10. For example, electrodecatheter 14 may include a handle portion, a fluoroscopy imaging device,and/or various other controls, to name only a few examples. Suchcomponents are well understood in the medical devices arts and thereforefurther discussion herein is not necessary for a complete understandingof the invention.

The exemplary system 10 may include a generator 40, such as, e.g., ACcurrent generator and/or a radio frequency (RF) generator, which in thepresent embodiment provides an electrical signal(s) to the electrode(s)of the catheter 14 as well as to the force transducer 22 (as illustratedby wires 44). A measurement circuit 42 is electrically connected to thetip electrode 20 and force transducer 22. The measurement circuit 42 maymeasure any appropriate electrical responses associated with theelectrodes and/or force transducer 22. The invention is not limited touse with any particular type or configuration of measurement circuit.

In an exemplary embodiment, the measurement circuit 42 may beoperatively associated with an elasticity determination module 50 whichmay include a processor and memory. The module 50 is operative toanalyze the response from the force transducer 22 and displacementinformation to determine an elasticity value. In relation to thedisplacement information, such information may be provided by a roboticactuator or a position sensor associated with the catheter.Alternatively, such information may be input by a user. The module 50may also generate displacement control signals to control thedisplacement of the catheter (e.g., by a robotic actuator) for forcemeasurement purposes.

The module 50 may utilize sets of force and displacement values todetermine the elasticity values for probed tissue locations. Suchelasticity values may alternatively be stored in memory, e.g., as tablesor other suitable data structures. The module 50 may then access thetables or equations in memory and determine an elasticity value a probedtissue location based on force and displacement information. The modulemay also correlate the elasticity value and location with an imageprovided on user output display 11.

FIG. 7 illustrates a protocol 100 that may be utilized to determineelasticity values for internal patient tissue. Initially, a contactsurface of a catheter is disposed (110) at an initial position relativeto a first location of an internal patient tissue area. Typically, suchan initial position is a contact position where the tissue is notcompressed or deflected. However, the initial contact position mayinclude an initial compression. Once disposed (110) at the initialposition, the contact surface is advanced (120) a predetermined distanceto a first measurement position relative to the tissue. At thismeasurement position, the tissue is compressed or deflected and therebyapplies a resistive force to the contact surface. At a predeterminedtime after advancement, a force measurement is obtained (130). Ifanother of force measurement is desired, the contact surface is againadvanced (120) a predetermined distance to another measurement positionrelative to the tissue. Likewise, another force measurement is obtained(130) this process may be repeated until a predetermined number of forcemeasurements for a corresponding displacements are obtained. Theacquired data, including the force measurements and tissue displacementvalues associated advancement of the contact surface, are utilized togenerate (140) an indication of tissue elasticity. Once such anindication is generated, the indication of elasticity for the firsttissue location may be correlated (150) with an image that may be outputto a user. If elasticity information for additional tissue locations isdesired, the process may be repeated for additional locations.

Although various embodiments of this invention have been described abovewith a certain degree of particularity, those skilled in the art couldmake numerous alterations to the disclosed embodiments without departingfrom the spirit or scope of this invention. For example, differentarrangements exist for determining the force between a contact surfaceof a catheter and patient tissue. Further, additional arrangements existfor determining the displacement of the catheter and/or patient tissue.An important feature of this invention is the correlation of the forceand displacement to generate an indication of tissue elasticity and useof this information for performing a medical procedure. Further it willbe appreciated that all directional references (e.g., upper, lower,upward, downward, left, right, leftward, rightward, top, bottom, above,below, vertical, horizontal, clockwise, and counterclockwise) are onlyused for identification purposes to aid the reader's understanding ofthe present invention, and do not create limitations, particularly as tothe position, orientation, or use of the invention. Joinder references(e.g., attached, coupled, connected, and the like) are to be construedbroadly and may include intermediate members between a connection ofelements and relative movement between elements. As such, joinderreferences do not necessarily infer that two elements are directlyconnected and in fixed relation to each other. It is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative only and not limiting.Changes in detail or structure may be made without departing from thespirit of the invention as defined in the appended claims.

1-28. (canceled)
 29. A method, comprising: receiving a first forcesignal from a catheter as the catheter is displaced relative to aninternal tissue location; receiving a first displacement signalindicative of movement of the catheter between a reference position andthe internal tissue location; and processing said first force signal andsaid first displacement signal to generate a tissue characteristic valuefor the internal tissue location.
 30. The method of claim 29, furthercomprising: selecting a level of ablation power based at least in parton said tissue characteristic value; and providing said level ofablation power to an ablation electrode disposed proximate to a distalend of the catheter.
 31. The method of claim 30, wherein said level ofablation power is selected at least in part to control a lesiondimension created at the internal tissue location by said providing saidablation power.
 32. The method of claim 29, wherein generating saidtissue characteristic value comprises generating a tissue elasticityvalue for the internal tissue location.
 33. The method of claim 29,wherein generating said tissue characteristic value comprises generatingan ablation lesion characterization for the internal tissue location.34. The method of claim 29, further comprising: receiving a second forcesignal from the catheter as displaced relative to the internal tissuelocation; receiving a second displacement signal indicative of movementof the catheter between the reference position and the internal tissuelocation; and wherein processing comprises processing said first andsecond force signals and said first and second displacement signals togenerate said tissue characteristic value.
 35. The method of claim 29,further comprising: based on said tissue characteristic valueidentifying a structure of interest proximate to the internal tissuelocation.
 36. The method of claim 35, wherein identifying a structure ofinterest comprises: identifying an area of ablated tissue proximate tothe internal tissue location.
 37. The method of claim 29, whereinreceiving said first displacement signal comprises identifying responsesof one or more sensors attached to the catheter to a signal transmittedform an external tissue location.
 38. The method of claim 29, whereinsaid receiving of said first force signal comprises: receiving force anddirection information, wherein said direction information is indicativeof a contact angle between the catheter and the internal tissuelocation.
 39. A method, comprising receiving position signals fromsensors disposed proximate to a distal end of a catheter while thecatheter is disposed relative to an internal tissue location; generatingcatheter displacement values based on said position signals, whereinsaid catheter displacement values are indicative of movement of thecatheter relative to a reference position; receiving a contact signalassociated with a contact surface of the catheter contacting theinternal tissue location; generating a force value, based on at leastone of said contact signal and one or more of said displacement values,said force value indicative of a force applied to the contact surface bythe internal tissue location; selecting a level of ablation power basedat least in part on said force value; and providing said level ofablation power to an ablation electrode disposed proximate to the distalend of the catheter.
 40. The method of claim 39, further comprising:correlating said force value with catalog data; and determining a tissuecharacteristic for the internal tissue location, wherein said level ofablation power is selected based at least in part on said tissuecharacteristic.
 41. The method of claim 40, wherein determiningcomprises determining a tissue elasticity value for the internal tissuelocation.
 42. The method of claim 40, wherein determining comprisesdetermining an ablation characteristic for the internal tissue location.43. The method of claim 39, wherein said receiving said contact signalfurther comprises receiving vector information indicative of a contactangle between the contact surface of the catheter and the internaltissue location.
 44. A method, comprising: receiving position signalsfrom sensors disposed proximate to a distal end of a catheter while thecatheter is disposed relative to an internal tissue location; generatingcatheter displacement values based on said position signals, whereinsaid catheter displacement values are indicative of movement of thecatheter relative to a reference position; receiving a multiple axiscontact signals associated with a contact surface of the cathetercontacting the internal tissue location; generating vector informationindicative of a force and direction of said force applied to the contactsurface of the catheter, wherein said vector information is generatedusing said multiple axis contact signals and one or more of saiddisplacement values; and altering an operation setting for the catheterbased at least in part on said vector information.
 45. The method ofclaim 44, wherein altering the operation setting for the cathetercomprises providing a level of ablation power to an ablation electrodedisposed proximate to the distal end of the catheter, wherein the levelof ablation power is selected based at least in part on said vectorinformation.
 46. The method of claim 44, wherein receiving multiple axiscontact signals comprises receiving a signal from a multi-axistransducer.
 47. The method of claim 44, wherein receiving multiple axiscontact signals comprises receiving a signals from at least two singleaxis transducers.
 48. The method of claim 44, further comprising:correlating said vector information with catalog data; and determining atissue characteristic for the internal tissue location based on saidcorrelating.
 49. The method of claim 48, wherein determining comprisesdetermining a tissue elasticity value for the internal tissue location.50. The method of claim 48, wherein determining comprises determining anablation characteristic for the internal tissue location.